1. Field of the Invention
The present invention relates to apparatus for controlling the rotational speed of an induction motor by changing the voltage applied to the motor.
2. Description of the Prior Art
It is known in the art that the voltage applied to an induction motor may be changed by controlling the firing angle of a thyristor connected between the induction motor and its power source whereby the rotational speed of the motor may be controlled. This, however, is impractical unless the load is of a positive polarity, i.e., the motor is required to produce a power-running torque to support such a load. In other words, if the load is of a negative polarity, i.e., the motor is required to produce a braking torque to support such a load, it is virtually impossible to control the speed of the load. This negative load appears in practice when a load such as, for example, an elevator cage, descends in its full load state. In such an event, as is known in the art direct-current power is usually supplied to the windings of the motor to establish a braking torque. (This is called DC braking.) The DC power is consumed as heat in the motor, resulting in an undesirable addition of heat to the motor.